Neuregulin 1 type III (Nrg1III) overexpression worsens the radial sorting defects of Lama2−/− mice.
(A) Transverse semithin sections of sciatic nerves from mice of the indicated genotypes at postnatal day 16 (P16) are shown. Nrg1IIItg//Lama2−/− mice have more bundles of naked axons (unsorted bundles) than Lama2−/− mice (arrowheads). (B) Electron micrograph analysis shows that in Lama2−/− and Nrg1III+/−//Lama2−/− nerves these unsorted bundles contain amyelinated, naked axons with diameter >1 μm (asterisk), while in wild-type (WT) and Nrg1IIItg nerves, there are only Remak bundles that contain axons ensheathed and smaller than 1 μm (arrow). (C) Number of unsorted bundles per nerve cross semithin section, showing a 3-fold increase in the number of unsorted bundles in Nrg1IIItg//Lama2−/− (106.33 ± 5.7 versus 35.33 ± 0.5 Lama2−/− ***p = 0.0005 by Student t test; n = 3). (C’): The number of Remak bundles on ultrathin electron microscopy (EM) sections is decreased in Nrg1IIItg//Lama2−/− and Lama2−/− mutants (***p = 0.001 by 1-way ANOVA with Bonferroni multiple comparison test, n = 4). (C”): All mutant nerves have reduced numbers of axons in Remak bundles on ultrathin EM sections (**p = 0.005; ***p = 0.001 by 1-way ANOVA with Bonferroni multiple comparison test, n = 4). (D) Examples of precocious myelination of axons that have not been sorted into a 1:1 relationship (arrows). Bar = 10 μm in A, 2 μm in B and D. The numerical data used in C–C” are included in S1 Data.